1. Field of the Invention
This invention relates to optical devices and graded refractive index lenses.
2. Discussion of the Related Art
A graded refractive index (GRIN) lens has a refractive index whose value varies with radial distance from the axis of the lens. The non-trivial variation in refractive index causes light refraction and gives the GRIN lens focussing capabilities that are similar to those of an ordinary lens. Therefore, many optical devices employ GRIN or ordinary lenses interchangeably.
Many optical devices use lenses to focus, collimate, or expand light beams. FIG. 1 shows a fiber device 10 in which a GRIN fiber lens 11 is fused to a terminal end 12 of an optical fiber 13. The GRIN fiber lens 11 expands and collimates the light beam emitted by the optical fiber 13. The GRIN fiber lens 11 improves the optical coupling between optical fiber 13 and fiber device 15 as compared to the coupling that would otherwise exist between the fiber 13 and device 15 due to diffraction. The GRIN fiber lens 11 reduces diffraction losses when the optical fiber 13 is optically coupled to another optical fiber.
Since the diameter of a light beam varies along the axis of a GRIN lens, the beam diameter variations provide a measure of the lens"" length. The length over which the variations in the beam diameter make two complete cycles is known as the pitch of the lens. Typically, lengths of GRIN lens are referred to in multiples of the pitch length, e.g., xc2xd pitch or xc2xc pitch.
Some optical devices embodying principles of the inventions include a GRIN fiber lens in which the refractive index has a new radial profile. When attached to an end of an optical fiber, the new GRIN fiber lens increases the Rayleigh range of the emitted beam above that of a light beam emitted by a similar fiber attached to a conventional GRIN lens. The increased Rayleigh range improves beam collimation so that the fiber may couple to other optical devices over larger distance ranges.
In first aspect, the invention features a method for fabricating a GRIN fiber. The method includes providing a doped silica-glass preform having an axial core and a tubular cladding surrounding and adjacent the core. The core has a graded index of refraction. The cladding has a lower refractive index than the core. The method includes etching the preform to remove an outer layer of the tubular cladding and drawing the GRIN fiber from the etched preform.
In a second aspect, the invention features another method for fabricating a GRIN fiber. The method includes forming a tube of silica-glass having a tubular core and a concentric tubular cladding adjacent and external to the tubular core. The core has a dopant density with a radially graded profile. The method also includes partially collapsing the tube by applying heat thereto. The partially collapsed tube has a central channel. The method includes passing a glass etchant through the central canal to remove an internal layer of silica glass, and then, collapsing the etched tube to a rod-like preform.